Anisotropic 2D materials are promising building blocks for future photonic and optoelectronic devices due to their low structural symmetry and in-plane optical anisotropy. This review systematically summarizes the crystalline structure, growth dynamics, optical anisotropy and their modulation strategies, and the corresponding photonic applications for emerging anisotropic 2D materials. First, the physical properties and crystalline structures of typical anisotropic 2D materials are briefly introduced. After that, special attention is paid to the growth mechanism of low-symmetry lattices, where the competition between different growth modes determines the crystal morphologies. Then, the physical principles of anisotropic optical absorption, photoluminescence, Raman scattering, photodetection, and nonlinear response are discussed based on recent scientific advances. The discussion on the techniques to modify the intrinsic in-plane anisotropy, along with the possibility of introducing optical anisotropy to the isotropic materials, add a new degree of freedom to the control over their optical properties. The review of application prospects also helps bridge the gap between the scientific exploration of novel anisotropic materials and the development of polarization-sensitive photonic devices. The discussions in this review will push forward the scientific frontier in the crystalline growth and anisotropy control of anisotropic 2D materials.

Lifting the valley degeneracy in two-dimensional transition metal dichalcogenides could promote their applications in information processing. Various external regulations, including magnetic substrate, magnetic doping, electric field, and carrier doping, have been implemented to enhance the valley splitting under the magnetic field. Here, a phase engineering strategy, through modifying the intrinsic lattice structure, is proposed to enhance the valley splitting in monolayer WSe 2 . The valley splitting in hybrid H and T phase WSe 2 is tunable by the concentration of the T phase. An obvious valley splitting of ∼4.1 meV is obtained with the T phase concentration of 31% under ±5 T magnetic fields, which corresponds to an effective Landeǵ eff factor of −14, about 3.5-fold of that in pure H-WSe 2 . Comparing the temperature and magnetic field dependent polarized photoluminescence and also combining the theoretical simulations reveal the enhanced valley splitting is dominantly attributed to exchange interaction of H phase WSe 2 with the local magnetic moments induced by the T phase. This finding provides a convenient solution for lifting the valley degeneracy of two-dimensional materials.

Inspired by the profound physical connotations and potential application prospects of the valleytronics, we design a two-dimensional (2D) WS2/h-VN magnetic van der Waals (vdW) heterostructure and study the control of valley degree of freedom through the first-principles calculations. A considerable spin splitting of 627 meV is obtained at the K valley, accompanied with a strong suppression of that at the K' valley. An intrinsic large valley splitting of 376 meV is generated in the valence band, which corresponds to an effective Zeeman magnetic field of 2703 T. Besides of the valence band, the conduction band of WS2 possesses a remarkable spin splitting also, and valley labelled dark exciton states are present at the K' valley.The strengths of spin and valley splitting relied on the interfacial orbital hybridization are further tuned continually by the in-plane strain and interlayer spacing. Maximum spin and valley splitting of 654 and 412 meV are finally achieved, respectively, and the effective Zeeman magnetic field can be enhanced to 2989T with a -3% strain. Time-reversal symmetry breaking and the sizable Berry curvature in the heterostructure lead to a prominent anomalous Hall conductivity at the K and K' valleys. Based on these finding, a prototype filter device for both the valley and spin is proposed.

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Two unequal K and K' valleys in transition metal dichalcogenides (TMDC) enable large and controllable polarization, which is the cornerstone of emerging valleytronic applications. Here, a phase engineering strategy aided by resonant plasmonic coupling is proposed to manipulate the valley degree of freedom. Compared with the pristine WSe2 monolayer, the hybrid H/T phase WSe2 exhibits an enhanced degrees of circular polarization (DCP) and valley polarization (DVP). As further aided by the designed Au plasmonic array, the T phase facilitates the excitons process and promotes the charge transfer in WSe2/Au interface under the plasmonic‐enhanced electromagnetic field. Consequently, both the DCP and DVP values are considerably enhanced to 38.5% (15.6%) and 15.1% (7.6%) at 13 K (room temperature), respectively. Through finite difference time domain simulations (FDTD), the near‐field excitation, exciton decay, and far‐field detection processes are systematically analyzed, and highly consistent polarizations are quantitatively achieved between the theoretical and the experimental results. Accordingly, the high polarizations are revealed to be contributed by the increased exciton generation and radiation efficiency, chiral electromagnetic field, and non‐equilibrium spin distribution in the hybrid phase. The research presented here illustrates a promising route to control the spin and valley degrees of freedom in TMDC materials.